GROUND WATER ATLAS of the UNITED STATES
Alabama, Florida, Georgia, South Carolina
HA 730-G

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SURFICIAL AQUIFER SYSTEM

INTRODUCTION

The surficial aquifer system (fig.
15) in the southeastern United States includes any otherwise
undefined aquifers that are present at the land surface. Even
though the sand and gravel aquifer of Florida and southwestern
Alabama, and the Biscayne aquifer of southern Florida are present
at the land surface and are the lateral equivalents of the surficial
aquifer system, they are treated separately in this Atlas because
of their importance as water sources. The sand and gravel, and
the Biscayne aquifers supply large municipalities; the surficial
aquifer system, although used by a large number of people, principally
is used only for domestic, commercial, or small municipal supplies.

The thickness of the surficial aquifer system is typically
less than 50 feet, but its thickness in Florida is as much as
400 feet in Indian River and St. Lucie Counties; 250 feet in Martin
and Palm Beach Counties; and 150 feet in eastern St. Johns County.
In southeastern Georgia, thicknesses of about 60 feet have been
mapped for the system. The system generally thickens coastward.

HYDROGEOLOGIC UNITS

The surficial aquifer system consists mostly of beds of unconsolidated
sand, shelly sand, and shell. Locally, in southwestern Florida,
limestone beds form an important and highly permeable part of
the system. In places, clay beds are sufficiently thick and continuous
to divide the system into two or three aquifers; mostly, however,
the system is undivided. Complex interbedding of fine- and coarse-textured
rocks is typical of the system.

The rocks that comprise the surficial aquifer system range
from late Miocene to Holocene in age. Although figure
16 shows that nine geologic formations are part of the system
at different places in Florida, the entire sequence of formations
is not present at any one location. The formations are thin and
mostly lens-like, and it is unusual for more than three or four
of them to comprise the aquifer system at any place. Many of the
geologic formations shown interfinger with each other, and some
of them, such as the Caloosahatchee Marl, are not particularly
productive aquifers. In Georgia and South Carolina, unnamed, sandy,
marine terrace deposits of Pleistocene age and sand of Holocene
age comprise the system. These sandy beds commonly contain clay
and silt. In Alabama, a thin, unnamed sand of Holocene age comprises
the system.

Limestone beds of the Tamiami and Fort Thompson Formations,
mostly restricted to southern and southwestern Florida, are the
most productive parts of the surficial aquifer system. Yields
from these formations are especially large where large-scale openings
have been developed by dissolution of part of the limestone. In
places where the combined Pamlico Sand and overlying sand deposits
of Holocene age are 40 feet or more thick, moderate yields are
obtained; elsewhere, the system generally does not yield much
water.

GROUND-WATER FLOW

Ground water in the surficial aquifer system is under unconfined,
or water-table, conditions practically everywhere. Locally, thin
clay beds create confined or semiconfined conditions within the
system. Most of the water that enters the system moves quickly
along short flowpaths and discharges as baseflow to streams.

The general movement of water within the system is illustrated
in figure 17, which is an idealized
diagram representing hydrologic conditions in Indian River County,
Fla. Water enters the system as precipitation. A large percentage
of this water is returned to the atmosphere by evapotranspiration.
Water that is not returned to the atmosphere by evapotranspiration,
or that does not directly run off into surface-water bodies, percolates
downward into the surficial aquifer system and then moves laterally
through the system until it discharges to a surface-water body
or to the ocean.

In places, some water leaks upward from the underlying Floridan
aquifer system through the clayey confining unit separating the
Floridan and surficial systems (fig.
17). In other places, where the hydraulic head of the Floridan
is lower than the water table of the surficial aquifer, leakage
can occur in the opposite direction.

Because the surficial aquifer system extends seaward under
the Atlantic Ocean, saltwater can encroach into the aquifer in
coastal areas. Encroachment is more extensive during droughts
because there is less freshwater available in the surficial aquifer
system to keep the saltwater from moving inland.

The configuration of the long-term, average water table of
the surficial aquifer system, where it has been mapped in the
eastern and southern part of the Florida peninsula, is shown in
figure 18. The water-table configuration
is generally a subdued reflection of the topography of land surface.
Steep gradients occur between streams and ridges or hills, and
gentle gradients occur in broad, flat interstream areas and under
broad topographic highs.

The arrows in figure 18 show that
the general direction of ground-water movement is toward the Atlantic
Ocean, the Gulf of Mexico, or toward major rivers. The water-table
surface is complex, reflecting the fact that water in the surficial
aquifer system moves quickly toward the nearest surface-water
body. Accordingly, local directions of ground-water movement change
markedly within short distances.

The wide spacing of the contours in Collier County and adjacent
areas reflects two conditions: (1) the Big Cypress Swamp, which
is virtually flat, is present throughout much of this area; and
(2) the surficial aquifer system largely consists of highly permeable
limestone in this area. Steeper gradients elsewhere are more typical
of a sand aquifer in an area of gentle topography.

The transmissivity of the surficial aquifer system is extremely
variable. Most reported values range from 1,000 to 10,000 feet
squared per day; in places, values of 25,000 to 50,000 feet squared
per day have been reported. The larger values are primarily for
beds of shell or limestone. Well yields range from less than 50
gallons per minute in most of Georgia and South Carolina, to 450
gallons per minute in St. Johns County, Fla., to 1,000 gallons
per minute in Indian River County, Fla.

FRESH GROUND-WATER WITHDRAWALS

Water-use data are available for the surficial aquifer system
only from Florida. About 361 million gallons per day of freshwater
was withdrawn from the surficial aquifer system in Florida during
1985. Nearly equal volumes were withdrawn for public supply and
for domestic and commercial uses (fig.
19), with withdrawals for these categories being about 154
and 157 million gallons per day, respectively. Agricultural withdrawals
accounted for about 13 million gallons per day, and withdrawals
for industrial, mining, and thermoelectric-power uses were about
4 million gallons per day, primarily for industrial use.